(i) Field of the Invention
This invention relates to hydraulic energy storage systems and, more particularly, relates to hydraulic energy storage systems used in vehicles to provide higher efficiency, smaller package size, lower weight, unitary construction, durability and enhanced reliability while maintaining the capability to efficiently store and restore energy at high power levels.
(ii) Description of the Related Art
Vehicles equipped with hydraulic energy storage systems have the ability to store kinetic energy while braking, rather than dissipate it through the brakes, and then restore it for subsequent acceleration. Such vehicles are commonly called “Hydraulic Hybrid” when the vehicle prime mover also contributes to the energy store, or “Stored Hydraulic Energy Propulsion” (SHEP) when only the vehicle energy is stored. This application refers to SHEP storage, but the inventions disclosed herein may be equally applicable to hydraulic hybrid vehicles.
The improvements of the present invention apply to hydro-pneumatic accumulators that are normally used to store energy in SHEP vehicles, Hydraulic Hybrids and to the associated hydraulic circuitry. In line with industry practice, the term “fluid” as used in this application refers to hydraulic fluid, typically a liquid such as a specially formulated mineral oil The term “gas” refers to the gas used to precharge a hydro-pneumatic accumulator, typically being dry nitrogen.
The performance and fuel economy of a vehicle, particularly one subject to frequent stops and starts, can be improved by recovering and storing the vehicle kinetic energy during deceleration and then restoring it, less any losses that may occur, during subsequent acceleration. SHEP systems have a hydraulic pump/motor (P/M) that can be connected to the drive train of the vehicle, so that the vehicle can be decelerated by pumping high pressure hydraulic fluid into a hydro-pneumatic accumulator thereby recovering the vehicle's kinetic energy. Subsequent acceleration can, at least in part, be achieved by using the stored kinetic energy to drive the P/M as a motor. Hydraulic hybrid systems have this same capability with the addition of a hydraulic pump driven by the vehicle engine. This provides a more flexible system at the cost of increased complexity. Importantly it provides for still further improvements in fuel economy by optimising engine usage.
Hydraulic hybrid and SHEP vehicles have been the subject of many patents and technical papers. U.S. Pat. No. 3,903,696 shows a basic SHEP system, with U.S. Pat. No. 4,760,697 being a more complex version, and U.S. Pat. No. 4,242,922 describing the basics of a hydraulic hybrid, all incorporated herein by reference.
Published technical papers covering the use of SHEP and hybrid systems in automobiles, buses, garbage trucks, trains and other vehicles are typified by the following papers: Mechanical power regeneration system; “Simulation of a Hydraulic Hybrid Vehicle Power Train”, ASME-Paper n 73-ICT-50, Sep. 23–27, 1973; “Practical Considerations for Energy-Storage Motor Vehicles”, published by ASME, New York, N.Y., U.S.A. 1981; and “Studies of an Accumulator Energy-Storage Automobile Design with a Single Pump/Motor Unit, SAE Paper 851677 1985.